Magnetron

ABSTRACT

Methods and devices for producing plasmas of more uniform density and greater height than plasmas generated by previously known magnetron-type plasma-generating devices. The present invention utilizes electrodes containing multiple magnets positioned such that like magnetic poles of the magnets are all facing in substantially the same direction.

This application is a Divisional of prior application Ser. No.08/936,153, filed Sep. 24, 1997, now U.S. Pat. No. 5,900,284.

This invention relates to plasma-generating devices. More specifically,this invention relates to magnetron-type plasma-generating devices(i.e., magnetrons) capable of sustaining plasmas of more uniform densitythan plasmas generated by previously known magnetron-typeplasma-generating devices.

Magnetrons have been known in the art for a long time and have beenused, for example, in etching, surface modification, and plasma-enhancedchemical vapor deposition ("PECVD"). PECVD devices are also known in theart. Examples of PECVD devices can be found in U.S. Pat. Nos. 5,298,587;5,320,875; 5,433,786; and 5,494,712, the teachings of which are hereinincorporated by reference.

As explained in Chapter 6 of the Handbook of Plasma ProcessingTechnology, Noyes Publications 1990, magnetrons are a class of coldcathode discharge devices generally used in a diode mode. A plasma isinitiated between the cathode and the anode at pressures in the mTorrrange by the application of a high voltage, which can be either dc orrf. The plasma is sustained by the ionization caused by secondaryelectrons emitted from the cathode due to ion bombardment which areaccelerated into the plasma across the cathode sheath. Whatdifferentiates a magnetron cathode from a conventional diode cathode isthe presence of a magnetic field. The magnetic field in the magnetron isoriented such that a component of the magnetic field is parallel to thecathode surface. The local polarity of the magnetic field is orientedsuch that the E×B drift paths of the emitted secondary electrons form aclosed loop. Due to the increased confinement of the secondary electronsin this E×B drift loop compared to a dc or rf diode device, the plasmadensity is much higher, often by an order of magnitude or more, than aconventional rf or dc diode plasma. The result of the high plasmadensity and its proximity to the cathode is a high current, relativelylow voltage discharge.

It is also known in the art, that when using a magnetron in a process tocoat a substrate such as in a PECVD process, it is difficult to obtain acoating of uniform thickness and quality. One aspect of quality isuniform chemical composition of the coating both in thickness and widthdirections. To get a coating of uniform thickness and quality thesubstrate must be moved relative to the electrodes. This is especiallytrue for large substrates. Moving the substrates relative to theelectrodes causes a decrease in throughput.

SUMMARY OF THE INVENTION

The present invention allows for more uniform (thickness and quality)coatings to be obtained more easily than do devices of the prior art,especially on large substrates. In one aspect, the present invention isan electrode to containing multiple magnets positioned such that likemagnetic poles of said magnets are all facing in substantially the samedirection. Each magnet produces a magnetic field between the oppositemagnetic poles on the same magnet. Each magnetic field has a componentparallel to the surface of the electrode. Electrodes of the presentinvention have a higher number of closed loop E×B drift paths per numberof magnets than electrodes of the prior art. Electrodes of the presentinvention are capable of producing a more uniform plasma across thesurface of an electrode. In addition, electrodes of the presentinvention produce plasmas of greater height than electrodes of the priorart. According to the present invention, large numbers of magnets (i.e.,two or more) can be aligned in various configurations so as to createlarge electrodes capable of producing large, more uniform, plasmas.

In another aspect, the present invention is an improvedplasma-generating device utilizing electrodes of the present invention.In another aspect, the present invention is an improved method offorming a plasma and an improved method for coating various substrates.For example, electrodes of the present invention can be advantageouslyutilized with the teachings of U.S. Pat. Nos. 5,298,587; 5,320,875;5,433,786; and 5,494,712 to produce improved PECVD devices and methodsof forming plasmas and coatings onto various substrates.

In one embodiment of the present invention, the electrode is a planarelectrode comprising two or more magnets positioned such that like polesof said magnets are in a single geometric plane parallel to thegeometric plane of the planar electrode and the polarity of said magnetsis perpendicular to the geometric plane of the planar electrode, eachmagnet producing a magnetic field having a component parallel to thegeometric plane of the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a plasma apparatus of the presentinvention.

FIG. 2 is an exploded view of an electrode of the present invention.

FIG. 3 is another view of the electrode of FIG. 2.

FIG. 4 is a view of another electrode of the present invention.

FIG. 5 is a view of an alignment of magnets useful in an electrode ofthe present invention.

FIG. 6 is a view of another alignment of magnets useful in an electrodeof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an apparatus of the present invention in which anelectrode of the present invention can be effectively utilized. Theapparatus comprises a reactor vessel 10 into which gaseous reactants canbe introduced from sources 11, 12, 13, and 14 through mass flowcontrollers 15, 16, 17, and 18. If desired, the different gases andvapors from the indicated sources can be mixed in a mixer 19 prior tointroducing them into the reactor vessel.

Disposed in the reactor vessel 10 are a pair of opposed electrodes 20and 21. The substrate to be treated is placed between the electrodes 20and 21. Electrode 21, the cathode, is connected to a variable frequencypower source 22. Electrode 20 can be advantageously grounded through thereactor vessel walls. Gaseous reactants are dispersed into the interiorof the vessel from gas supply line 23. The reactor vessel 10 can beadvantageously connected to a vacuum system 24 for evacuating the vessel10. Optionally, the reactor vessel could be equipped with monitoringdevices such as an optical monitor 25 to determine the thickness of thecoating.

Preferably, both electrodes 20 and 21 are embodiments of the presentinvention. However, it is not necessary for both electrodes to beembodiments of the present invention. If only one electrode is anembodiment of the present invention, then preferably electrode 21, thecathode, is an embodiment of the present invention.

In operation, the reactor vessel 10 is first evacuated by means of thevacuum pump 24 prior to introducing gaseous reactants (e.g.,organosilicone and oxygen) and inert gases, if any, to the vessel at apredetermined flow rate through supply line 23. When the flow rate ofthe gases becomes constant the variable frequency power 22 is turned onto a predetermined value to generate a plasma which causes the reactantsto form a film on the substrate.

FIG. 2 depicts an exploded view of an electrode 21 of the presentinvention. In FIG. 2, bar magnets 30 are placed on a bottom plate 32.The magnets 30 can be advantageously adhered to the bottom plate 32. Aheader plate 31 is place on the magnets 30. The header plate 31 mayoptionally have holes or slits in it to allow the gaseous reactants andinert gases, if present, to pass through it as taught in U.S. Pat. No.5,433,786.

It is a key feature of the present invention that each magnetic pole ofeach magnet in an electrode of the present invention produces a magneticfield with the opposite magnetic pole on the same magnet. This isdepicted in FIG. 3, wherein the north pole of each magnet 30 forms amagnetic field 33 with the south pole of the same magnet. Each magnet 30has a component 34 of the magnetic field 33 that is parallel to theelectrode surface. Each magnet that forms a magnetic field with theopposite magnetic pole of the same magnet creates at least one closedloop E×B drift path.

Thus, magnets in electrodes of the present invention are configured suchthat each magnet creates its own closed loop E×B drift path(s).Preferably, this is done by aligning the magnets in electrodes of thepresent invention so that like magnetic poles are all facing insubstantially the same direction, as shown in FIG. 3. By substantiallythe same direction, it is meant that like magnetic poles of all magnetsare facing in the same direction relative to the surface of theelectrode. Thus, although the electrode depicted in FIG. 3 is a planarelectrode, it is envisioned that the electrode could be curved. Forexample, the electrode could be formed into a cylindrical shape with allnorth poles facing outward from the center of the cylinder.

When magnets in electrodes of the present invention are aligned so thattheir like magnetic poles are facing in substantially the samedirection, the magnets can be positioned in close proximity of eachother without having any of the magnets forming magnetic fields with anyof the magnets placed near it. Thus, even when magnets are placed inclose proximity to each other, each magnet still creates its own closedloop E×B drift path(s).

Because the magnets can be placed in close proximity with each otherwhile each magnet maintains its own closed loop E×B drift path(s),electrodes of the present invention enjoy the benefit of having moreclosed loop E×B drift paths per electrode surface area. This increasednumber of closed loop drift paths per electrode surface area results ina more uniform plasma than plasmas produced using magnetic confined typeelectrodes of the prior art. It has also been visually observed thatplasmas generated using electrodes of the present invention diffusefarther away from the electrode surface in the space between theelectrodes than plasmas generated using electrodes of the prior art.Although it is not definitively known exactly why this behavior isobserved, it is believed that a portion of each magnetic field isrepelled away from the electrode surface by the like magnetic poles onthe electrode surface. It is also believed that these portions ofmagnetic field result in a portion of the plasma to be produced furtherfrom the electrode surface than would be possible if the magnets werepositioned with alternating polarity.

Large electrodes of the present invention can be created by configuringlarge numbers of magnets, all having like magnetic poles facing insubstantially the same direction. For example, FIG. 4 shows a planarelectrode of the present invention containing two rows of magnets 30.Even larger electrodes can be produced by increasing the number ofmagnets in each row or by adding more rows of magnets.

When bar magnets are utilized according to the teachings of the presentinvention as shown in FIG. 4, each magnet 30 creates a single closedloop E×B drift path 35. However, if circular magnets are utilized asshown in FIG. 5 or FIG. 6, each magnet creates two separate closed loopE×B drift paths. For example, FIG. 5 shows concentric rings of magnets50 wherein the visible surface of each magnet (i.e., the flat surfacefacing the reader) has the same polarity. Each magnet in FIG. 5generates two separate closed loop E×B drift paths. FIG. 6 showscircular magnets 60 all having the same radius aligned in a cylindricalshape. If the curved surface of each magnet 60 that faces outward fromthe center of the cylinder has the same polarity then each magnet 60will generate two separate closed loop E×B drift paths.

The magnets utilized in electrodes of the present invention should notbe positioned so close to each other so as to prevent one magnetic poleof the magnet from producing a magnetic field with the opposite magneticpole of the same magnet. If the magnets are positioned too close to eachother they may behave as a single magnet. There is no criticallimitation as to how far apart the magnets may be positioned. However,as magnets are positioned farther apart, their corresponding closed loopE×B drift paths are farther apart and the resulting plasma produced willbe less uniform than a plasma produced when the magnets are positionedcloser together. A distance between magnets adequate for a givenapplication can be determined without undue experimentation.

Wider bar magnets will produce a larger gap in the center of thecorresponding closed loop E×B drift path. Larger gaps in the closed loopE×B drift paths also result in the production of a less uniform plasma.Thus, it is generally more desirable to utilize relatively narrow barmagnets. However, if the magnets utilized in electrodes of the presentinvention are too narrow then the closed loop E×B drift path also willbe too narrow, making the plasma increasingly difficult to initiate. Amagnet width adequate for a given application can be determined withoutundue experimentation.

EXAMPLES

Deposition of SiO_(x) C_(y) H_(z), of was carried out according to theteachings of U.S. Pat. No. 5,433,786 except that the deposition wascarried out in a PECVD stainless steel box equipped with a pair ofelectrodes of the present invention. Each electrode was a planarelectrode having dimensions of 30 inches by 120 inches. Each electrodewas constructed of 5 segments, each segment having dimensions of 30inches by 24 inches. Each segment was constructed by arranging 2 rows of12 bar magnets on a backing plate made of a 1/16 inch soft iron sheet.The magnets were arranged in the manner depicted in FIG. 4. Each of themagnets was 8.5 inches long, 0.75 inches wide, and 0.5 inches high. Themagnets in each row were placed 1.5 inches apart Each magnet had asurface field of 1 kilogauss. The magnets were obtained from MidwestIndustries. On one electrode each magnet was placed such that the northpole of each magnet faced away from the backing plate and on the otherelectrode each magnet was placed such that the south pole of each magnetfaced away from the backing plate. The magnets were covered with a 3/16inch aluminum sheet (header plate) which became the surface of theelectrode. The electrodes were placed in the PECVD chamber in parallel 9inches apart so that the surface of one electrode (all north poles)faced the surface of the other electrode (all south poles). Utilizingthese two electrodes, the PECVD device generated uniform plasmaconditions over an area approximately 30 inches by 120 inches.

What is claimed is:
 1. A plasma-generating device comprising twoelectrodes, at least one of said electrodes defining an electrodesurface and containing three or more magnets, each magnet in themagnet-containing electrode having two opposite magnetic poles, allmagnets in the magnet-containing electrode positioned such that likemagnetic poles of said magnets are all facing in substantially the samedirection, each magnetic pole of each magnet producing a magnetic fieldwith the opposite magnetic pole on the same magnet; each magnetic fieldhaving a component parallel to said electrode surface.
 2. Aplasma-generating device according to claim 1, wherein each magnetcreates a single closed loop E×B drift path parallel to the surface ofthe electrode.
 3. A plasma-generating device according to claim 1,wherein each magnet creates two closed loop E×B drift paths parallel tothe surface of the electrode.
 4. A plasma-generating device comprisingtwo electrodes, at least one electrode being a planar electrode andcontaining three or more magnets, each magnet in the planar,magnet-containing electrode having two opposite magnetic poles, allmagnets in the planar, magnet-containing electrode positioned such thatlike magnetic poles of said magnets are in a single geometric planeparallel to the geometric plane of the planar electrode and the polarityof said magnets is perpendicular to the geometric plane of the planarelectrode, each magnetic pole of each magnet producing a magnetic fieldwith the opposite magnetic pole on the same magnet, each magnetic fieldhaving a component parallel to the geometric plane of the electrode. 5.A plasma-generating device according to claim 4, wherein each magnetcreates a single closed loop E×B drift path parallel to the geometricplane of the planar electrode.
 6. A plasma-generating device accordingto claim 4, wherein each magnet creates two closed loop E×B drift pathsparallel to the geometric plane of the planar electrode.
 7. Aplasma-generating device comprising:a) two electrodes, at least one ofsaid electrodes defining an electrode surface and containing three ormore magnets, each magnet in the magnet-containing electrode having twoopposite magnetic poles, all magnets in the magnet-containing electrodepositioned such that like magnetic poles of said magnets are all facingin substantially the same direction, each magnetic pole of each magnetproducing a magnetic field with the opposite magnetic pole in the samemagnet, each magnetic field having a component parallel to the electrodesurface, the magnets having sufficient strength to generate at least 100gauss; and b) a means for injecting gaseous reactants through at leastone electrode, said means directing substantially all of said reactantsthrough the magnetic fields.
 8. A plasma-generating device according toclaim 7, wherein each magnet creates a single closed loop E×B drift pathparallel to the surface of the electrode.
 9. A plasma-generating deviceaccording to claim 7, wherein each magnet creates two closed loop E×Bdrift paths parallel to the surface of the electrode.
 10. A method ofproviding an abrasion resistant coating onto the surface of a substrateemploying plasma enhanced chemical vapor deposition of an organosiliconemonomer gas in a plasma reaction zone and oxygen gas, comprising thesteps of:a) plasma polymerizing the organosilicone monomer in thepresence of excess oxygen employing a power density within the range ofabout 10⁶ to about 10⁸ J/Kg in the presence of the substrate; and b)conducting the oxygen and organosilicone monomer gases in a directionwhich is essentially perpendicular to the substrate surface and througha magnetic field of at least 100 gauss, which is contained essentiallyin a zone adjacent to the plasma zone and into the plasma reactionzone,wherein the magnetic field of at least 100 gauss is produced by anelectrode, the electrode defining an electrode surface and containingthree or more magnets, each magnet in the electrode having two oppositemagnetic poles, all magnets in the electrode positioned such that likemagnetic poles of said magnets are all facing in substantially the samedirection, each magnetic pole of each magnet producing a magnetic fieldwith the opposite magnetic pole on the same magnet, each magnetic fieldhaving a component parallel to the electrode surface.
 11. Aplasma-generating device according to claim 10, wherein each magnetcreates a single closed loop E×B drift path parallel to the surface ofthe electrode.
 12. A plasma-generating device according to claim 10,wherein each magnet creates two closed loop E×B drift paths parallel tothe surface of the electrode.